Recent development of a fuel level sender with a resistor card inside a sealed chamber is described in U.S. Pat. No. 6,851,315, by Bergsma et al. Fuel and/or fuel vapor is prevented from entering the chamber, eliminating deleterious chemical interaction between fuel and materials of the sender card, e.g. silver and/or silver alloys. A driving source on the outside of the chamber comprises either a ferrous pole or a magnet magnetically coupled with a magnet inside the chamber, or a magnet coupled with ferrous material inside the chamber for turning a fan inside the chamber without physical contact with the drive source. The chamber has a non-magnetic wall separating the inside of the sealed chamber from the outside, and the wall is free of any hole or intrusion (except where one or more electric terminals protrude through in a sealed manner that maintains sealing of the chamber interior from liquid fuel and fuel vapor), and with no requirement for dynamic sealing of a rotary shaft penetrating the chamber wall to couple a driving source to an internal component. Cost and technical complexity of a shaft seal have prevented its use in a high-volume, low-cost, application such as an in-tank fuel level sender in a mass-produced motor vehicle. Magnetically coupled devices are well known for isolating a component such as a pumping blade in a liquid pump from an outside environment.
One of the earliest inventions for a magnetically coupled drive is U.S. Pat. No. 1,847,006, by Kalischer, 1932, followed by many other related inventions: U.S. Pat. No. 2,460,015, by Jones; U.S. Pat. No. 4,163,164 by Pieters; U.S. Pat. No. 5,090,944 by Kyo et al.; and more recently U.S. Pat. No. 6,417,591 by Saito, et al. and U.S. Pat. No. 6,908,291 by Klein, et al. The use of magnets and ferrous materials both internally and externally to provide a rotary magnetically coupled drive is well known to those skilled-in-the-art.
Torque generated by a magnetically coupled drive is a key specification parameter. The number of magnets, the material, the air gaps, and the flux closure paths are all important in a drive design. A most important design goal of a magnetically coupled drive is that its magnets and poles are aligned one-for-one between the inside and outside of the chamber. The wall thickness of the chamber has a major effect on the permissible air gap between the magnetic poles. The environment both inside and outside the chamber may determine the type of magnetic materials that can be used, as corrosion and chemical effects may affect one material versus another. Size and volume limitations for the device may limit the size and number of magnetic sources and flux closure paths. Some systems are designed with a single magnet on the outside of the chamber wall and a single magnet on the inside, both to one side of a pivot. Flux closure elements may not be part of the design.
A magnetic drive for external reading of a liquid level in a tank, the gauge sealed only against elements of the weather, is described in U.S. Pat. No. 6,089,086, by Swindler, et al., 2000. U.S. Pat. No. 6,564,632, by Ross, Jr., 2002 describes a hermetic gauge, again sealed only against the atmospheric environment. It should be noted that U.S. Pat. No. 6,089,086 limits its use of a magnetically coupled gauge to a gauge environment external to a tank; the gauge is not designed to be used in a tank with materials designed for sealing against liquids and vapor in the tank. In fact, the proposed magnetic structure is not desirable; it leads to a strong pull along the shaft length of the rotating axis. The cylindrical magnet and the pointer magnet are disposed at different axial distances from the tank or pointer, on opposite sides of a wall perpendicular to the axis of rotation. They pull at each other along the axial direction, creating significant friction in the bearing assembly including the pointer. A better arrangement is magnets positioned radially around ring structures both internally and externally. This prevents axial thrust. U.S. Pat. No. 6,417,591 reads, “Moreover, because force produced between the first magnet group and the second magnet group acts in the radial direction, but not in the thrust direction, there is no likelihood that bearings, etc. will be damaged by the force.” However, an unbalanced arrangement of magnets may also produce cross-thrusts that causes significant friction effects leading to incorrect angular positioning of the pointer assembly.